Variable displacement hydraulic pumps, such as axial piston variable displacement pumps, are used in a variety of applications to provide pressurized hydraulic fluid. For example, hydraulic construction machines, earth working machines, and the like, often use variable displacement hydraulic pumps to provide the pressurized hydraulic fluid flow required to perform desired work functions.
Operationally, as the torque load on the engine of such a machine increases, the engine speed will decrease. When the torque load on the engine exceeds the engine's torque capabilities, the engine speed will be lugged down. If this lugging phenomenon progresses, the engine will stall. To avoid engine stalling, the torque load on the engine is desirably limited within the engine capability. Therefore, controlling and limiting the overall torque load on the engine is a very important machine control.
It is difficult for a hydro-mechanical control system design to provide a pump control system that maintains the total torque load of a plurality of pumps within a predetermined total torque load limit. Conventionally, a very conservative approach is used to limit the torque loads of all of the pumps in the pump system to the same level. By this way, some approximation will be implemented by a well-tuned hydro-mechanical controller, which is imposed on each pump. In addition to the conservativeness, this kind of controller has other drawbacks. First, the cost is high for hydro-mechanical control systems. A complicated hydro-mechanical system involve many machine parts with very fine manufacturing requirements. Additional cost for hydraulic routing and manifolds can also be associated with this control design. Second, much of the work in hydro-mechanical control design for variable displacement pumps uses linear control techniques. This means that the pump system dynamics are first linearized around an operating point and a controller is then synthesized for the linear system. However, control strategies that rely on linearizing a nonlinear system require good models of the system for stable precision-control and can result in a limited operating range. Third, setting the displacement of all the pumps to the same value to obtain torque-limiting control can cause discontinuity in pump control commands. The discontinuities can cause the machine operation to change abruptly or induce instability.